Balloon catheter tapered shaft having high strength and flexibility and method of making same

ABSTRACT

A catheter having an elongated shaft with a tubular member which forms at least a portion of the shaft and which is formed of a biaxially oriented thermoplastic polymeric material, and a method of forming the catheter shaft by radially and longitudinally expanding the tubular member to biaxially orient the polymeric material, and, in one embodiment, tapering the tubular member to form a bending stiffness transition. A catheter of the invention has an improved combination of low bending stiffness, high rupture pressure, and high tensile strength, for improved catheter performance.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.11/480,143, filed Jun. 30, 2006, now U.S. Pat. No. 7,906,066,incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates generally to medical devices, and particularly tointracorporeal devices for therapeutic or diagnostic uses, such asballoon catheters.

BACKGROUND OF THE INVENTION

In percutaneous transluminal coronary angioplasty (PTCA) procedures, aguiding catheter is advanced until the distal tip of the guidingcatheter is seated in the ostium of a desired coronary artery. Aguidewire, positioned within an inner lumen of a dilatation catheter, isfirst advanced out of the distal end of the guiding catheter into thepatient's coronary artery until the distal end of the guidewire crossesa lesion to be dilated. Then the dilatation catheter having aninflatable balloon on the distal portion thereof is advanced into thepatient's coronary anatomy, over the previously introduced guidewire,until the balloon of the dilatation catheter is properly positionedacross the lesion. Once properly positioned, the dilatation balloon isinflated with fluid one or more times to a predetermined size atrelatively high pressures (e.g. greater than 8 atmospheres) so that thestenosis is compressed against the arterial wall and the wall expandedto open up the passageway. Generally, the inflated diameter of theballoon is approximately the same diameter as the native diameter of thebody lumen being dilated so as to complete the dilatation but notoverexpand the artery wall. Substantial, uncontrolled expansion of theballoon against the vessel wall can cause trauma to the vessel wall.After the balloon is finally deflated, blood flow resumes through thedilated artery and the dilatation catheter can be removed therefrom.

In such angioplasty procedures, there may be restenosis of the artery,i.e. reformation of the arterial blockage, which necessitates eitheranother angioplasty procedure, or some other method of repairing orstrengthening the dilated area. To reduce the restenosis rate and tostrengthen the dilated area, physicians frequently implant a stentinside the artery at the site of the lesion. Stents may also be used torepair vessels having an intimal flap or dissection or to generallystrengthen a weakened section of a vessel. Stents are usually deliveredto a desired location within a coronary artery in a contracted conditionon a balloon of a catheter which is similar in many respects to aballoon angioplasty catheter, and expanded to a larger diameter byexpansion of the balloon. The balloon is deflated to remove the catheterand the stent left in place within the artery at the site of the dilatedlesion. Stent covers on an inner or an outer surface of the stent havebeen used in, for example, the treatment of pseudo-aneurysms andperforated arteries, and to prevent prolapse of plaque. Similarly,vascular grafts comprising cylindrical tubes made from tissue orsynthetic materials such as polyester, expanded polytetrafluoroethylene,and DACRON may be implanted in vessels to strengthen or repair thevessel, or used in an anastomosis procedure to connect vessels segmentstogether.

In the design of catheter shafts, strength, stiffness and flexibility ofvarious sections of the catheter shaft are specifically tailored toprovide the desired catheter performance. However, one difficulty hasbeen optimizing the often competing characteristics of strength andflexibility of the catheter shaft.

Accordingly, it would be a significant advance to provide a catheterhaving a catheter shaft with an improved combination of characteristicssuch as strength, flexibility and ease of manufacture. This inventionsatisfies these and other needs.

SUMMARY OF THE INVENTION

This invention is directed to a catheter having an elongated shaft witha tubular member which forms at least a portion of the shaft and whichis formed of a biaxially oriented thermoplastic polymeric material. Oneaspect of the invention is directed to a method of forming the cathetershaft by radially and longitudinally expanding the tubular member tobiaxially orient the polymeric material. Additionally, one aspect of theinvention is directed to forming a taper along at least a section of thebiaxially oriented tubular member to provide a bending stiffnesstransition. A catheter of the invention preferably has an improvedcombination of low bending stiffness, high rupture pressure, and hightensile strength, for improved catheter performance.

A method of making a catheter shaft of the invention generally comprisesradially and longitudinally expanding an extruded tube, which results inan expanded tubular member having a higher rupture pressure and tensilestrength than a tube extruded directly to the same final dimensions(i.e., wall thickness and outer diameter) as the expanded tubularmember. It is believed that the radial and longitudinal expansioncircumferentially and longitudinally orients the polymeric structurewithin the material. However, the orientation does not significantlyincrease the bending stiffness of the tubular member. Thus, a relativelylow durometer polymer can be selected to minimize bending stiffness inthe radially and axially deformed tubular member. The inherently lowbending stiffness of the low durometer polymer provides a longitudinallyflexible shaft tubular member which more readily bends duringmaneuvering of the catheter within the patient.

In a presently preferred embodiment, the catheter is a balloon cathetergenerally comprising an elongated shaft having a proximal end, a distalend, an inflation lumen extending therein, and a tubular member whichhas the inflation lumen therein and which is formed of a biaxiallyoriented nonporous thermoplastic polymer, and a balloon sealinglysecured to a distal shaft section. In one embodiment, the balloon is arelatively high pressure balloon. The biaxially oriented polymer haspolymer chains oriented longitudinally along the tubular member forincreased tensile strength, and circumferentially around the tubularmember for increased rupture pressure. The high tensile strength of theshaft tubular member improves catheter performance by, for example,increasing the ability to safely pull the catheter from within thepatient's vessel without tearing apart/damaging the catheter, e.g.,during retrieval of the catheter lodged in a calcific lesion.

The balloon has an interior in fluid communication with the inflationlumen, and a rupture pressure which is significantly less than therupture pressure of the shaft tubular member. As a result, the ballooncatheter preferably has a failure mode in which the balloon will rupturebefore the pressure-containing catheter shaft tubular member, to preventor minimize vessel injury in the event of a catheter rupture. In oneembodiment, the balloon is a relatively high pressure balloon, forexample having a rupture pressure of at least about 20 atm or more. Theshaft tubular member preferably has a mean rupture strengthsubstantially greater than that of the balloon, so that the distributionof the two rupture pressure ranges have essentially no statisticaloverlap.

In a method of making a balloon catheter having an elongated shaft and aballoon on a distal shaft section, a thermoplastic polymeric materialhaving a relatively low Shore durometer hardness is melt-extruded toform a tube having a lumen and a first inner and outer diameter whichare smaller than the desired final dimensions of a shaft tubular member.The method includes cooling the extruded tube to a temperature less thanan elevated temperature of the melt-extrusion, and placing the extrudedtube in a lumen of a capture member, and biaxially orienting thepolymeric material of the extruded tube within the capture member at anelevated temperature. The tube is biaxially oriented by radiallyexpanding the heated extruded tube with pressurized media in the tubelumen and simultaneously or sequentially axially expanding the extrudedtube with a load applied on at least one end of the tube. The expandedtube is thus radially and axially expanded to a second (larger) outerand inner diameter and a second (longer) length. The second outerdiameter is generally about equal to the inner diameter of the capturemember, and the second inner diameter is preferably at least about 5times larger than the first inner diameter of the extruded tube. Theexpanded tube is then cooled to room temperature, to produce thebiaxially oriented nonporous thermoplastic polymer tubular member(hereafter, “the biaxially oriented tubular member”), which forms atleast a portion of the catheter shaft.

The amount of radial expansion is selected to produce a high degree ofcircumferential orientation, which results in a correspondingly highrupture pressure for use as a shaft section which contains the inflationlumen therein. Thus, the method includes sealingly securing a balloon toa distal end of the biaxially oriented tubular member, such that theballoon has an interior in fluid communication with the lumen (i.e., theinflation lumen) of the biaxially oriented tubular member duringcatheter assembly.

By extruding a low durometer thermoplastic material to form a tubehaving a significantly smaller inner diameter and larger wall thicknessthan the required shaft tubular member, and then radially andlongitudinally expanding the tube, a tubular member is provided whichhas a low bending stiffness but nonetheless has high rupture pressureand tensile strength. Moreover, the increased rupture pressure is notprovided at the expense of other performance characteristics of thecatheter. For example, although the rupture pressure of a tubular shaftcan be increased by increasing the wall thickness, the correspondingdecrease in the shaft inner and/or outer diameter disadvantageouslyincreases the inflation/deflation time and the profile of the shaft.

The Shore durometer hardness of the polymeric material, and the extrudedand expanded dimensions of the tubing are selected such that theresulting tubular member preferably has a Gurley bending stiffness valueof not greater than about 50 to about 150 mg, a rupture pressure of atleast about 25 to about 50 atm, and a tensile break load of at leastabout 1.0 to about 5.0 lbf. In a presently preferred embodiment, theShore durometer hardness of the polymeric material is about 63 D,although a polymeric material having a lower or higher Shore durometerhardness can alternatively be used. Polymeric materials found useful inthe invention typically have a Shore durometer hardness of about 55 D toabout 75 D.

In the design of shafts for balloon catheters, extruded catheter shafttubing is conventionally resized to a smaller diameter and wallthickness by necking the tubing using a die and mandrel. Unlike suchconventional necking procedures which force the tubing through a die andthus primarily elongate the tubing with only a minimal decrease intubing diameter and/or wall thickness, the catheter shaft tubing of theinvention is highly circumferentially oriented by being radiallyexpanded to an inner diameter significantly larger than the original(extruded) inner diameter. In one embodiment, the tubing is radiallyexpanded to substantially the maximum amount possible (based on thepolymeric material and extruded tubing dimensions), which results inexpanded tubing having minimal radial growth at increasing innerpressures. Consequently, the expanded tubing has an improved controlledfailure mode. In the event that the shaft tubing is over-pressurizedabove the rupture pressure of the shaft tubing, the expanded tubingpreferably fails by rupturing with a small longitudinally extending slitand without radially expanding against the vessel wall, which thusprevents or minimizes vessel injury.

In one embodiment, the biaxially oriented tubular member is providedwith a wall thickness and/or diameter that tapers along at least asection of the biaxially oriented tubular member, to thereby vary thebending stiffness therealong. In a presently preferred embodiment, thetapered section is formed during the biaxial orientation of the tube byvarying an external load applied on at least one end of the extrudedtube as a function of a heating nozzle position (i.e., as the heatingnozzle traverses along the length of the extruded tube during thebiaxial orientation expansion, the external axial load is varied, tovary the amount of axial expansion). The resulting tapered section ofthe biaxially oriented tubular member can have a variety of differentconfigurations depending on factors such as the amount by which theexternal load is varied and whether the capture member has a taperedinner diameter. In a presently preferred embodiment, the resultingtapered section of the biaxially oriented tubular member tapers distallyto a smaller outer diameter and wall thickness, but has a substantiallyconstant inner diameter. However, a variety of suitable configurationscan be used including a tapered section having a tapered inner diameterwith or without a tapered outer diameter and wall thickness.

Although discussed herein primarily in terms of a tapering method inwhich the extruded tube is tapered by varying the external axial loadduring the biaxial expansion of the tube, the taper can be formed usingadditional or alternative methods. For example, the internal pressureused to radially expand the extruded tube during the biaxial orientationalso exerts an axial force which can expand the tube in the axialdirection. Therefore, varying the internal gas pressure during biaxialorientation will change the overall axial load carried by the tube'scross section and thus the total axial stress arising during biaxialorientation. Additionally, the expanded tubular member's dimensions canbe modified after the biaxial expansion by radially shrinking thebiaxially oriented tubular member onto a mandrel during a heatstabilization process. Depending on the shape (e.g., tapering ornontapering) of the mandrel and the biaxially oriented tubular memberprior to the heat stabilization process, this radial shrinking canproduce a finished part having a constant or varying inner diameterand/or outer diameter. However, radial and axial shrinkage onto amandrel after the biaxial orientation produces a greater wall thickeningin sections having greater initial clearance between the biaxiallyoriented tubular member and the mandrel outer diameter. Thus, in onepresently preferred embodiment, the amount of shrinkage during the heatstabilization process is minimized, by heat stabilizing the biaxiallyoriented tubular member on a mandrel having an outer diameter profilewhich closely matches (i.e., little initial clearance within) the innerdiameter of the as-expanded biaxially oriented tubular member.

Although a taper can be produced using various methods, a preferredembodiment produces a relatively large stiffness transition, withoutsacrificing the desired degree of biaxial orientation, and without adisadvantageous decrease in the inner lumen diameter. In a presentlypreferred embodiment, the bending stiffness of the tapered biaxiallyoriented tubular member varies by a factor of about 2 to about 4.5 alongthe length of the taper. The stiffness transition is typically providedover a relatively long length. However, the biaxially oriented tubularmember can be provided with a stiffness transition due to the varyingwall thickness and/or diameter which is abrupt (e.g., occurring overabout 1 cm), less abrupt (e.g., occurring over a few to manycentimeters), or essentially continuous (e.g., occurring over all or asubstantial majority of the length of the tubular member). Generally,the tapered biaxially oriented tubular member has at least a sectionwhich has a distally tapering wall thickness and/or diameter along alength of at least about 1 cm, or about 2% of the total length of thebiaxially oriented tubular member.

The tapered biaxially oriented tubular member can be used in a varietyof suitable locations of a variety of catheters, although it istypically a relatively flexible distal section of the catheter shaft,with the tapered section enhancing the stiffness transition from ahighly flexible distal end to a substantially more rigid proximalsection of the catheter shaft. In a presently preferred embodiment, thestiffness transition of the tapered biaxially oriented tubular membereliminates the need for a separate mid-shaft section secured to thedistal end of a stiff proximal section and the proximal end of aflexible distal section of the shaft. The tapered biaxially orientedtubular member thus preferably provides improved catheter deliverabilitywithout increasing the number of shaft components or complicatingcatheter assembly.

The invention provides a catheter shaft tubular member having animproved combination of low bending stiffness, high rupture pressure,and high tensile strength. Preferably, a catheter shaft tubular memberof the invention has a low profile and high flexibility such that thecatheter has excellent ability to track and to cross tight, tortuousanatomy, while having a high rupture pressure and the ability tomaintain inflation lumen integrity during a medical procedure. The highrupture pressure catheter shaft assures that inadvertentover-pressurization will normally result in rupture within the balloon(and most notably even a relatively high rupture pressure balloon) atthe treatment site rather than elsewhere in the patient's vasculature.Unlike conventional catheter design in which shaft sections requiringminimized profile and/or maximized lumen size are typically formed ofhigh strength/stiffness materials to allow for the shaft to be formedwith thin walls, the catheter shaft section of the invention is formedof a relatively low durometer polymeric material providing a low bendingstiffness. Similarly, unlike shaft sections formed with multiple layersor reinforcements to increase the burst pressure/strength of the shaft,the catheter shaft section of the invention has relatively thin walls,for minimizing the shaft profile while maximizing the shaft lumen size,and for minimizing the shaft bending stiffness.

These and other advantages of the invention will become more apparentfrom the following detailed description and accompanying exemplarydrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a ballooncatheter embodying features of the invention.

FIGS. 2 and 3 are transverse cross sectional views of the ballooncatheter shown in FIG. 1, taken along lines 2-2 and 3-3, respectively.

FIG. 4 illustrates the formation of the catheter shaft outer tubularmember, in which an extruded tube is radially and longitudinallyexpanded in a capture member in a method embodying features of theinvention, with the extruded tube shown prior to being radially andlongitudinally expanded.

FIG. 5 illustrates the extruded tube of FIG. 4 after being radially andlongitudinally expanded in the capture member.

FIG. 6 is an elevational view, partially in section, of an over-the-wirestent delivery balloon catheter embodying features of the invention,which has a tapered biaxially oriented tubular member forming a portionof the catheter shaft.

FIG. 7 is a longitudinal cross sectional view of the catheter shaft ofFIG. 6, taken along line 7-7.

FIG. 8 is a transverse cross sectional view of the balloon cathetershown in FIG. 6, taken along line 8-8.

FIG. 9 a illustrates a method of forming a tapered biaxially orientedtubular member, with an extruded tube shown prior to being radially andlongitudinally expanded within a tapered capture tube.

FIG. 9 b illustrates the extruded tube of FIG. 9 a after being radiallyand longitudinally expanded in the capture member.

FIG. 10 illustrates a tapered biaxially oriented tubular member having asection with a tapered inner and outer diameter.

FIG. 11 illustrates a tapered biaxially oriented tubular member having asection with a tapered inner diameter, and a uniform outer diameter.

FIG. 12 illustrates the tapered biaxially oriented tubular member ofFIG. 11 after being heat stabilized on a uniform diameter mandrel.

FIG. 13 illustrates the second order polynomial fit to total axialstress as a function of axial stretch percentage, for experimentally andanalytically derived data for biaxially oriented tubing according to amethod embodying features of the invention.

FIG. 14 is a graph of the predicted moment of inertia as a function ofnozzle position for various tapered biaxially expanded tubular members.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a rapid exchange type balloon dilatation catheter 10embodying features of the invention. Catheter 10 generally comprises anelongated catheter shaft 11 having a proximal end 12, a distal end 13, aproximal shaft section 16, and a distal shaft section 17, and aninflatable balloon 14 on the distal shaft section. The shaft 11 has aninflation lumen 20, and a guidewire receiving lumen 21. An adapter 18 atthe proximal end of the catheter provides access to the inflation lumen20 and is configured for connecting to an inflation fluid source (notshown). The distal end of the catheter may be advanced to a desiredregion of a patient's body lumen in a conventional manner and balloon 14inflated to perform a medical procedure such a dilate a stenosis, andcatheter 10 withdrawn or repositioned for another procedure. FIG. 1illustrates the balloon inflated. FIGS. 2 and 3, illustrate transversecross sections of the catheter of FIG. 1, taken along lines 2-2 and 3-3,respectively.

In the illustrated embodiment, the proximal shaft section 16 comprises aproximal tubular member 22 defining a proximal portion of the inflationlumen 20, and the distal shaft section 17 comprises a distal outertubular member 23 defining a distal portion of the inflation lumen 20,and an inner tubular member 24 defining the guidewire lumen 21configured to slidably receive guidewire 26 therein. As a rapid exchangetype catheter, the guidewire lumen 21 extends from a distal port 27 atthe distal end of the catheter to a proximal port 28 spaced distallyfrom the proximal end of the catheter. The rapid exchange junction atthe guidewire proximal port 28 is the transition between the singlelumen proximal shaft section and the multilumen distal shaft section inthe illustrated embodiment. Similarly, in one embodiment, the guidewireproximal port 28 is located in a midshaft section extending between andconnecting the proximal tubular member 22 and the distal outer member23. The distal shaft section is preferably more flexible than theproximal shaft section, and the proximal tubular member is thereforetypically a relatively high stiffness material such as a metal or highdurometer polymer. As best illustrated in FIG. 2, the inflation lumen 20in the distal shaft section is the annular space between the innersurface of the outer tubular member 23 and the outer surface of theinner tubular member 24, although a variety of suitable shaftconfigurations can alternatively be used including non-coaxial andmulti-lumen extrusions.

Balloon 14 is sealingly secured to the shaft such that the ballooninterior is in fluid communication with the shaft inflation lumen 20.Specifically, in the illustrated embodiment, the balloon 14 has aproximal skirt section bonded to the distal end of shaft distal outertubular member 23 and a distal skirt section bonded to the distal end ofshaft inner tubular member 24. The balloon 14 is preferably formed of apolymeric material which is compatible with the material forming theouter surface of the shaft, to allow for fusion bonding, although theballoon can alternatively or additionally be adhesively bonded to theshaft. The balloon 14 is preferably a relatively high rupture pressure,non-compliant balloon, which in one embodiment has a rupture pressure ofabout 20 to about 30 atm, such that the balloon can be inflated in thepatient during a procedure at relatively high working pressure of about18 atm. In one embodiment, the balloon has a rated burst pressure ofabout 14 to about 25 atm. The rated burst pressure (RBP), calculatedfrom the average rupture pressure, is the pressure at which 99.9% of theballoons can be pressurized to without rupturing, with 95% confidence.Generally, a balloon 14 is inflated in the patient during a procedure atworking pressure of about 8 to about 18 atm.

In accordance with the invention, at least a portion of the cathetershaft 11 comprises a tubular member formed of a biaxially orientedthermoplastic polymeric material, which in the illustrated embodimentpreferably is the distal outer tubular member 23 (hereafter “thebiaxially oriented distal outer tubular member”) having the inflationlumen 20 therein. A catheter of the invention can have a biaxiallyoriented tubular member alternatively or additionally forming othersections of the catheter shaft including proximal and midshaft sections.However, unlike the proximal shaft section which is typically formed ofa relatively high bending stiffness material to provide sufficient push(force transmission) for advancing the catheter in the patient'svasculature, the distal shaft section preferably has tubular memberswith a low bending stiffness to provide sufficient flexibility to trackover a guidewire in the patient's distal tortuous vasculature.

The polymeric material of the biaxially oriented distal outer tubularmember 23 is biaxially oriented by radially and longitudinally expandingan extruded tube used to form the distal outer tubular member 23, asdiscussed in more detail below.

The biaxially oriented distal outer tubular member 23 is formed of arelatively soft/low durometer polymeric material. The polymer preferablyhas a Shore durometer hardness of not greater than about 63 D to about70 D. A variety of suitable nonporous polymeric materials can be usedincluding polyether block amide (PEBAX) copolymers, polyurethanes,polyethylenes, and polyesters. The polymeric material can have variouslevels of crystallinity, and thus can be crystalline or noncrystalline.In a presently preferred embodiment, the polymer is a single polymer orcopolymer (i.e., not a blend of two separate polymers). For example, apresently preferred polymer is PEBAX 63D, which has a Shore durometerhardness of about 63 D.

In a presently preferred embodiment, the distal outer tubular member 23is a single-layered tubular member (i.e., not a multi-layered tube),formed of the biaxially oriented polymer tubing. The biaxially orienteddistal outer tubular member 23 thus does not require multiple layers ofdifferent polymeric materials or reinforcements to provide the desiredcombination of characteristics. Additionally, unlike porous polymericmaterials which are expanded during processing to produce a desiredporosity, the biaxially oriented distal outer tubular member 23 isitself fluid tight (i.e., non-porous) and thus does not require anon-porous additional layer in order to hold the inflation fluid. Thus,due to the nature of the thermoplastic polymeric material, the tubularmember formed therefrom is not porous, and the radial and longitudinalexpansion does not render the tubular member porous. A single-layeredtubular member provides ease of manufacture and avoids problemsassociated with multi-layered shafts such as layer delamination andthickness nonuniformities.

In the illustrated embodiment, the biaxially oriented distal outertubular member 23 has a uniform outer diameter along the entire lengthof the tubular member 23. In one embodiment, the biaxially orienteddistal outer tubular member 23 has an inner diameter of about 0.028 toabout 0.029 inches, and an outer diameter of about 0.0325 to about0.0335 inches along at least a section thereof. The length of thebiaxially oriented distal outer tubular member 23 is typically about 15to about 20 cm.

The rupture strength of the catheter shaft is important for insuringthat the balloon 14 can be inflated to the desired high pressure duringa medical procedure. If the relatively low durometer polymeric materialwas extruded to the final (expanded) dimensions of the biaxiallyoriented distal outer tubular member 23, the resulting tubular memberwould have a rupture strength which would be significantly lower thanthe desired value, and, for example, which would not be significantlygreater than the balloon 14 rupture pressure. In the catheter 10 of theinvention, the balloon rated burst pressure is significantly less than(e.g., about 4 atm less than, or about 20% less than) that of thebiaxially oriented tubular outer member 23.

FIGS. 4 and 5 illustrate a method of making a biaxially oriented tubularmember such as the biaxially oriented distal outer tubular member 23 ofthe catheter 10 of FIG. 1. A method of the invention generally comprisesmelt-extruding a thermoplastic polymeric material having a relativelylow Shore durometer hardness, to form a tube 30 having a lumen 31, afirst inner and outer diameter (ID₁, OD₁) and a first length (L₁), andcooling the extruded tube 30 to a temperature (e.g., to roomtemperature) which is less than an elevated temperature of themelt-extrusion. The cooled extruded tube 30 is placed within a capturemember 32, heated to an elevated temperature, and radially and axiallyexpanded in the capture member 32 to a second inner and outer diameter(ID₂, OD₂) and length (L₂), to thereby biaxially orient the polymericmaterial of the extruded tube 30. FIG. 4 illustrates the extruded tube30 disposed within the capture member 32 prior to being expandedtherein, and FIG. 5 illustrates the expanded tube 30′ within the capturemember 32 (i.e., the extruded tube 30 of FIG. 4 after being radially andlongitudinally expanded within the capture member 32). After beingradially and longitudinally expanded, the resulting expanded tube 30′ iscooled to room temperature and heat stabilized as discussed in moredetail below. The catheter 10 is subsequently assembled, at least bysealingly securing a balloon to a distal end of the expanded tubularmember such that the balloon has an interior in fluid communication withthe expanded tubular member lumen.

The dimensions of the extruded tube 30 are set by the extrusionapparatus, and are typically not resized (e.g., hot die necked) prior tothe radial and longitudinal expansion. For example, the tubular memberis typically extruded by a screw extruder having a die and mandrel sizedso that upon ordinary draw-down the tubular member exiting the extruderhas the first outer diameter (OD₁), and the first inner diameter (ID₁).

In the embodiment of FIG. 4, the capture member 32 is tubular with aninner surface layer 33 of a lubricious polymeric material such aspolytetrafluoroethylene (PTFE) for subsequent ease of part removal,reinforced with an outer high strength jacket layer 34 such as stainlesssteel tubing configured to prevent or inhibit diameter creep (growth)after repeated use. Thus, the capture member 32 is configured toradially restrain the growing tube 30, without the inner or outerdiameter of the capture member 32 increasing at the elevated internalpressures used to radially expand the extruded tube 30.

The extruded tube 30 is heated to the elevated temperature within thecapture member 32, which in the illustrated embodiment comprisesdirecting heat from a heating nozzle 35 at the outer surface of thecapture member 32. In a presently preferred embodiment, the heatingnozzle 35 traverses along a length of the extruded tube 30, from a firstend to the opposite end. Thus, the radial and longitudinal expansion isinitiated with only the first end of the extruded tube 30 heated by thenozzle 35 in one embodiment. In a presently preferred embodiment, theextruded tube 30 is heated to an expansion elevated temperature which isless than the melt-extrusion elevated temperature (i.e., less than amelting temperature of the polymeric material).

The extruded tube 30 is axially expanded with a load applied on at leastone end of the tube, e.g., using a vertical necking apparatus (notillustrated), and is radially expanded with pressurized media introducedinto the extruded tube lumen from a pressurized media source (notillustrated) connected to one end of the extruded tube 30. Specifically,with the heating nozzle 35 heating the first end of the extruded tube30, the heating nozzle 35 is moved toward the second end and the load isapplied to the second end in the same direction as the heating nozzlemovement to axially expand (i.e., stretch lengthwise) the extruded tube30. The amount of the load required to provide the desired stretchpercent depends on factors such as the tensile elongation, dimensions,material of the tubing 30, pressure of the pressurized media, and theexpanded inner diameter. The pressurized media, e.g., compressed air, isat an elevated pressure sufficient to initiate the radial expansion,such that the wall hoop stress exceeds the material resistance(typically the yield stress) to stretching at the blowing temperature.The internal pressure used to radially expand the tubing 30 is typicallyabout 400 to about 600 psi.

The extruded tube 30 is preferably simultaneously radially and axiallyexpanded at the elevated temperature, for ease of manufacture. However,it can alternatively be sequentially expanded (i.e., first radially thenlongitudinally, or first longitudinally and then radially).

The tubing 30 is preferably radially expanded into contact with theinner surface of the capture member 31, to the second outer diameterwhich is about equal to the inner diameter of the capture member 31. Thetubing 30 radially expands in all directions around the tubingcircumference, resulting in circumferential orientation of the polymericmaterial. In a presently preferred embodiment, the second inner diameter(ID₂) is at least about 5 times larger than the first inner diameter(ID₁) of the extruded tube (i.e., the blow-up-ratio, BUR, of theexpanded tubular member 30′ is at least about 5, and is morespecifically about 5.8 to about 6). The large BUR provides a high degreeof circumferential orientation, for a large increase in the rupturepressure of the tubing. In one embodiment, the tubing is radiallyexpanded to substantially the maximum amount possible (i.e., to a BURwhich is at least about 80% of the maximum BUR possible). Specifically,as the tubing radially expands, the radius increases and the tubing wallthickness decreases, which results in a rapid increase in the wall hoopstress during constant pressure blowing. If the wall hoop stress of thegrowing tubing exceeds the ultimate hoop strength of the material,rupture will occur. As a result, there is a limit to the BUR (i.e., amaximum attainable BUR) of a polymeric material forming the tubing. Theresulting expanded tubular member 30′ exhibits little additional radialexpansion at increasing internal pressures and preferably has a rupturemode consisting of a small longitudinally extending slit, for minimalvessel injury in the event of a shaft rupture. Within the workingpressure range of the balloon 14, the biaxially oriented distal outermember 23 preferably has minimal radial growth, and as the pressure isincreased above the rated burst pressure, the orientation preferablyprevents the formation of a bulbous, highly expanded pocket along thebiaxially oriented distal outer member 23 which can otherwise form as anouter member wall expands as the pressure therein approaches the rupturepressure.

Although the dimensions will vary depending upon the type of catheterand desired use of the biaxially oriented tubular member, the extrudedfirst inner diameter (ID₁) is generally about 0.004 to about 0.006inches and the extruded first outer diameter (OD₁) is generally about0.021 to about 0.023 inches, whereas the expanded second inner diameter(ID₂) is generally about 0.028 to about 0.029 inches and the expandedsecond outer diameter (OD₂) is generally about 0.0325 to about 0.0335inches.

The dimensions of the expanded tube 30′ are typically stabilized afterthe radial and longitudinal expansion using a heat stabilization processin which the expanded tube 30′ is heated for a duration at an elevatedtemperature sufficient to stabilize the polymeric material of the tube.In a presently preferred embodiment, the heat stabilization comprisesheating the expanded tube 30′ on a mandrel which controls the amount ofradial shrinkage. Specifically, the expanded tube 30′ is placed on amandrel and reheated to a temperature above room temperature buttypically below the expansion temperature to allow for radial recoveryonto the mandrel and for the radial and axial dimensions to stabilize.The mandrel outer diameter is slightly smaller than the inner diameterof the expanded tubular member 30′, to allow for slidably mounting theexpanded tubular member 30′ thereon. The amount of radial and axialshrinkage is relatively minimal, i.e., not greater than about 5%, andthe heat stabilization preferably does not substantially decrease therupture pressure of the tubular member. The heat stabilizationtemperature is typically significantly more than the polymeric glasstransition temperature but less than the elevated temperature usedduring the radial and axial expansion. In a presently preferredembodiment, a PEBAX tubular member is heat stabilized at about 100 toabout 140° C. for about 10 to about 15 minutes.

In one embodiment, the biaxial orientation of the polymer of the tubularmember 30′ is substantially uniform along the entire length thereof.Thus, the extruded tube 30 is radially expanded by a substantiallyuniform amount along the length thereof, and is longitudinally expandedby a substantially uniform amount, to produce an expanded tube 30′having a substantially uniform inner and outer diameter along the lengththereof. For example, in the embodiment illustrated in FIGS. 4 and 5,the capture member 32 has a uniform inner diameter configured toradially restrain the expanding extruded tube 30 at the second outerdiameter, such that the second outer diameter of the expanded tube 30′is uniform along the length of the expanded tube 30′. In an alternativeembodiment, the inner diameter of the capture member varies along atleast along a section of the capture member 32, to produce a taperedbiaxially oriented tubular member, as discussed in more detail below.Ruler markings on the ends of the extruded tube 30 can be comparedbefore and after the longitudinal expansion to confirm that the desiredoverall stretch percent is achieved. The amount of longitudinalexpansion, expressed as a stretch percent, typically ranges from about50 to about 200% of the initial length (L₁) of extruded tube 30. In theembodiment in which the expanded tube 30′ has a substantially uniforminner and outer diameter along the entire length thereof, the axialstretch percentage is, more specifically, preferably about 75 to about100% of the initial length (L₁).

The final expanded dimensions (ID₂, OD₂) are preferably predicted andcontrolled during formation of the expanded tubular member 30′, tothereby provide a desired bending stiffness, rupture strength, tensilebreak load, and percent elongation to failure. During the radial andaxial expansion, the inner diameter of the extruded tubing 30 increasesdue to both the internal pressure and the longitudinal stretching. Thus,extruded tubes having different wall thicknesses can be expanded tosimilar final expanded dimensions (ID₂, OD₂) using the same capturemember 32 by using different stretch percentages. Moreover, significantcharacteristics of the resulting expanded tubular member can be tailoredby selecting and controlling the nature of the extruded tube and amountof expansion. For example, the break load of the expanded tubular membercan be increased by increasing the outer diameter of the startingextrusion (OD₁) and correspondingly increasing the stretch percent. Theelongation to failure of the expanded tubular member can be increased byincreasing the elongation of the starting extrusion.

In the embodiment of FIG. 1, the biaxially oriented distal outer tubularmember 23 of catheter 10 has a uniform wall thickness and diameter (thebiaxially oriented tubular member 23 preferably being the section of theshaft outer member which is located distal to the rapid exchange notchat guidewire proximal port 28). In alternative embodiments, thebiaxially oriented tubular member is formed with at least a sectionhaving a tapering wall thickness and/or diameter.

FIG. 6 illustrates a balloon catheter 110 which has a tapered biaxiallyoriented distal outer tubular member 123 with a diameter and wallthickness that are distally tapering (i.e., decreasing in a distallyextending direction). The balloon catheter 110 generally comprises anelongated shaft 111 having a proximal end, a distal end, a guidewirelumen 21 and an inflation lumen 20 extending therein, an inner tubularmember 124 with the guidewire lumen 21 therein, and a proximal outertubular member 122 with a proximal portion of the inflation lumen 20therein, and a distal outer tubular member 123 which has a distalportion of the inflation lumen 20 therein and which is formed of atapered biaxially oriented nonporous thermoplastic polymer. A balloon 14sealingly secured to a distal shaft section has a proximal skirt securedto the distal outer tubular member 123 and a distal skirt secured to theinner tubular member 124 such that an interior of the balloon is influid communication with the inflation lumen 20. An adapter 118 on theproximal end of the shaft 111 is configured to provide access to theguidewire lumen 21 and has an arm 119 configured for connecting to asource of inflation fluid (not shown) for inflating the balloon 14. Inthe illustrated embodiment, the balloon catheter 110 has a stent 115releasably mounted on the balloon 14 for delivery and deployment withina patient's body lumen, although the catheter 110 can be configured fora variety of uses including angioplasty, drug delivery and the like. Thestent delivery balloon catheter 110 is an over-the-wire type catheter,and thus has the guidewire lumen 21 extending the entire length of thecatheter shaft 111 to a proximal port at the proximal end of thecatheter, unlike the rapid exchange type catheter 10 of the embodimentof FIG. 1 having a guidewire proximal port 28 distal to the proximal endof the catheter 10. The catheter 110 is otherwise similar to thecatheter 10 of the embodiment of FIG. 1, such that the discussion aboverelating to the biaxially oriented tubular member formed of a relativelylow durometer material yet having a relatively high rupture pressureapplies as well to the embodiment of FIG. 6.

The tapered biaxially oriented distal outer tubular member 123 has afirst (distal) section 130 with a uniform wall thickness and diameter,and a second (proximal) section 131 with a wall thickness and outerdiameter tapering distally from the proximal end of the distal outertubular member 123, as best illustrated in FIG. 7 showing an enlargedlongitudinal cross section taken within line 7-7 of FIG. 6.

The distal section 130 of the biaxially oriented distal outer tubularmember 123 extends from the tapering proximal section 131 to the distalend of the biaxially oriented distal outer tubular member, and has aconstant inner and outer diameter and a constant wall thickness alongsubstantially its entire length (i.e., constant within normalmanufacturing tolerances). The tapering proximal section 131 similarlyhas a constant inner diameter, such that the inner diameter is constantalong the entire length of the biaxially oriented distal outer tubularmember 123 in the embodiment of FIG. 6.

The proximal end of the tapering proximal section 131 is bonded,typically by fusion and/or adhesive bonding, to the distal end of aproximally adjacent section of the shaft (e.g., to the proximal outertubular member 122), and the tapered proximal section 131 extendsdistally of the bond. The tapered proximal section is generally about 2%to about 80% of the length of the tapered biaxially oriented tubularmember 123 of over-the-wire catheter 110, and specifically, in theembodiment illustrated in FIG. 7, the tapered proximal section 131 isabout 40% of the length of the tapered biaxially oriented tubular member123. However, the taper can be a more abrupt shorter taper, or a moregradual taper extending along a longer length or along essentially theentire length of the biaxially oriented tubular member 123.

The taper length of a tapered biaxially oriented tubular member wouldtypically be longer in an over-the-wire balloon catheter compared to arapid exchange balloon catheter due at least in part to the affect ofthe formation of the rapid exchange notch (i.e., at proximal guidewireport 28 in the embodiment of FIG. 1) of a rapid exchange catheter.Specifically, forming the rapid exchange notch typically requires heatand radially inward force to bond the tubular members together at theproximal guidewire port 28, which would change the biaxial orientationand tapering of the tubular member. As a result, in a rapid exchangecatheter, the biaxially oriented tubular member, and more specificallythe tapered section of the biaxially oriented tubular member, ispreferably located distal to the proximal guidewire port 28. Expressedas a percentage of the total length of the shaft 111, the length of thetapered section 131 is generally about 1% to about 20% of the totallength of the shaft 111 of the over-the-wire type catheter 110 of FIG. 6(from the proximal to the distal end of the catheter). In contrast, in arapid exchange type catheter such as catheter 10 of FIG. 1, a taperedsection of the biaxially oriented distal outer tubular member isgenerally about 1% to about 15% of the length of the shaft (or generallyabout 5% to about 90% of the length of the biaxially oriented distalouter tubular member 23 of rapid exchange catheter 10).

In a presently preferred embodiment, the catheter 110 does not have aseparate midshaft outer tubular member of intermediate stiffness totransition from the relatively stiff proximal shaft section to therelatively flexible distal shaft section. Rather, the tapered section131 of biaxially oriented outer tubular member 123 provides asufficiently large change in bending stiffness therealong that theproximal end of the biaxially oriented distal outer member 123 issecured directly to the distal end of the relatively stiff proximalouter member 122. The bending stiffness of the tapered biaxiallyoriented tubular member typically decreases distally along the length ofthe taper by about 20% to about 80%, more preferably by about 40% toabout 80% (the 80% decrease corresponding to a percent increase(proximally) of about 350%). Thus, the bending stiffness changes alongan integral, one-piece tubular member (tapered biaxially orientedtubular member 123), without requiring variable reinforcements such asbraids, coils, sleeves, or multiple layers along the tubular member 123to vary the bending stiffness therealong.

In a presently preferred embodiment, in order to provide a relativelylarge bending stiffness change along the length of the taper, the wallthickness in addition to the diameter of the biaxially oriented tubularmember varies along the length of the taper. Preferably, the wallthickness of the tapered biaxially oriented tubular member 123 increasesby about 50% to about 250% proximally along the tapered section 131, andas a result the bending stiffness of tapered tubular member 123increases proximally by at least about 65%, and more preferably about100% to about 350%, and more specifically by about 200% to about 350%therealong. For example, in one embodiment of tapered tubular member 123having an inner diameter of about 0.7 mm, the wall thickness alongconstant wall thickness section 130 is about 0.05 mm to about 0.06 mm,and the wall thickness increases to a maximum of about 0.14 mm to about0.16 mm at the proximal end section of the tapered section 131, suchthat the bending stiffness increases proximally by as much as about 340%(corresponding to a decreases distally of as much as about 78%). Theinner diameter of tubular member 123 is typically about 0.65 mm to about0.75 mm.

In a method of making a tapered biaxially oriented tubular member, theextruded tube is biaxially oriented as discussed above in relation tothe embodiment of FIGS. 4 and 5. However, in a presently preferredembodiment, at least a section of the extruded tube is tapered duringthe biaxial orientation, preferably by varying the overall axial loadthat produces the axial orientation of the polymeric material of thetube. Specifically, the extruded tube is placed in a capture membersimilar to the capture member 32 of FIG. 4, and biaxially oriented bybeing heated with an external heat supply traversing longitudinally froma first to a second section of the tube and radially expanded withpressurized media in the tube lumen, and simultaneously axially expandedwith an overall axial load that is controllably varied as at least asection of the tube is heated. As a result, different longitudinalportions along the length of a given biaxially oriented tubular memberare thereby axially stretched by different amounts. Generally, the axialstretch percentage of the different longitudinal portions of theresulting tapered biaxially oriented tubular member varies from about50% to about 300% of the initial length of the tube portion. Preferably,varying the overall axial load comprises varying an external load (e.g.,weight) applied on one end of the tube. However, in one embodiment, theoverall axial load is varied by additionally or alternatively varyingthe pressure of the pressurized media in the tube lumen.

In one presently preferred embodiment, the tapered biaxially orientedtubular member is biaxially oriented in a capture member which tapersfrom a larger to a smaller inner diameter along at least a section ofthe capture member. FIG. 9 a illustrates extruded tube 30, prior tobeing biaxially expanded, in a tapered capture tube 132 having a uniforminner diameter along a first section 133 extending from a first end ofthe capture member, and having a tapered inner diameter along a secondsection 134 extending from a second end of the capture member to theuniform diameter first section 133. The overall axial load is decreasedas the heat supply (typically a heating nozzle 35) traverses along thetapered section 134 of the capture member 132 from the smaller to thelarger inner diameter ends of tapered section 134.

Preferably, the inner diameter contour of the tapered capture membercorresponds to the desired expanded outer diameter profile of thetapered biaxially oriented tubular member. Thus, by increasing the innerdiameter of the capture tube 132 along section 134, a biaxially orientedtubular member can be formed which has a tapered outer diameter, such astubular member 123 of the embodiment of FIG. 6. In order to provide theconstant inner diameter of tapered biaxially oriented tubular member123, the external weight (or overall axial load) is constant as theheating nozzle 35 traverses along section 133, and is controllablyvaried by a particularly specified amount as the heating nozzle 35traverses along tapered section 134 of the tapered capture tube 132, andFIG. 9 b illustrates the resulting tapered biaxially expanded tube 30′within the capture member 132. As a result, although the extruded tube30 will radially expand to a larger diameter along tapered section 134of the tapered capture member 132, the corresponding thinning of itswall thickness which would otherwise occur is prevented by reducing theaxial stretch percentage. The resulting tapered biaxially orienteddistal outer member 123 has an axial orientation that increases distallyalong the tapered proximal section and has a circumferential orientationthat is substantially uniform along the entire length of the biaxiallyoriented tubular member 123 at least within a radial inner surfaceportion thereof. To produce the tapered biaxially oriented tubularmember 123 of the embodiment of FIG. 7, after expansion of the tube 30in the tapered capture member 132, the as-expanded tubular member istypically heat stabilized, preferably on a constant outer diametermandrel (see e.g., the mandrel 146 of FIG. 12) as discussed in moredetail below.

The axial external weight values which are required to prepare a desiredtapered biaxially oriented tubular member are predicted based oncalculated axial stretch percentages and calculated total axial stressvalues for an extruded tube 30 of a given as-extruded inner and outerdiameter. By selecting the varying external weight values applied to thetube 30, the resulting biaxially expanded tubular member can be causedto have a variety of different tapering configurations. For example, astube 30 is expanded in the tapered capture member 132, the externalweight decreases by a relatively large amount (see below Example 2) inorder to provide the tapered biaxially oriented tubular member 123 ofFIG. 7 having a tapering outer diameter and wall thickness but aconstant inner diameter. In contrast, if the external weight was insteaddecreased by a lesser amount in the tapered capture member 132, atapered biaxially oriented tubular member 135 could be prepared whichhas a tapering inner and outer diameter, as illustrated in FIG. 10. Inthe embodiment of FIG. 10, the tapered biaxially oriented tubular member135 has a proximal end 136, a distal end 137, a nontapering distalsection 138, and a tapering proximal section 139 with a distallytapering inner and outer diameter and a constant wall thickness. Taperedbiaxially oriented tubular member 135 of the embodiment of FIG. 10 istypically less preferred than the embodiment of FIG. 7 due to therelatively small change in bending stiffness readily produced withoutvarying the wall thickness of the tubular member 135. However, theincreased inner diameter along the tapered proximal section 139,relative to tapered proximal section 131 of the embodiment of FIG. 7, ispreferred in a catheter in which a large lumen size is more important tocatheter performance than a large change in bending stiffness providedby the tapered wall thickness of the biaxially oriented tubular member.

Thus, the biaxially oriented tubular member can be provided with avariety of different tapering configurations. For example, FIG. 11illustrates a tapered biaxially oriented tubular member 140 having afirst end 141, a second end 142, a nontapering first section 143, and atapering second section 144 with a constant outer diameter and an innerdiameter which tapers toward the second end 142 of the tubular member140. As a result, the wall thickness (and bending stiffness) increasesalong the length of section 144 from the nontapering first section 143to a larger wall thickness at the second end 142 of the tubular member140 (i.e., the wall thickness increases inversely to the decreasinginner diameter of the tapered biaxially oriented tubular member 140).The tapered tubular member 140 can be prepared in a uniform diametercapture member 32 during biaxial orientation of the extruded tube 30, byapplying a constant axial weight as the heating nozzle 35 traversesalong the first section of the extruded tube 30, and then graduallydecreasing the axial weight as the heating nozzle 35 traverses along thesecond section of the extruded tube 30.

By varying the tension along the length to form the taper during biaxialorientation, the method of the invention facilitates localizedcompensation for dimensional and property variability within theoriginal as-extruded tube 30 (i.e., adjusting for lot-to-lot materialproperty variation).

A tapered biaxially oriented tubular member of the invention generallyhas a bending stiffness that decreases by the relatively large amount ofa factor of about 2 to about 4 along the length of the tapering section,which is provided by varying the wall thickness and the diameter of thebiaxially oriented tubular member along the length of the taperingsection. The change in bending stiffness (moment of inertia) ispreferably linear, or substantially linear (i.e., within normalmanufacturing tolerance), and the uniform taper is specifically providedwith desired dimensions at a desired location along the biaxiallyoriented tubular member using a method of the invention.

As discussed above in relation to the embodiment of FIG. 1, a biaxiallyoriented tubular member of the invention is typically heat stabilizedafter the biaxial expansion by being heated on a mandrel. In oneembodiment, the method of producing the tapered biaxially orientedtubular member takes advantage of the diameter shrinkage that occursduring the heat stabilization. For example, provided the heatstabilization time and temperature are sufficient to radially shrink theexpanded tubular member by a desired amount, heat stabilizing thetapered biaxially oriented tubular member 140 of FIG. 11 on a constantdiameter mandrel 146 provides a resized tapered biaxially orientedtubular member 145 having a constant inner diameter and a variable outerdiameter and wall thickness, as illustrated in FIG. 12. However, theheat stabilization process induces both radial and axial shrinkage whichleads to greater wall thickening in sections having greater initialclearance between the as-expanded inner diameter and the mandrel outerdiameter. As a result, a tubular member that is biaxially expanded to aninner diameter profile substantially equal to the final target and heatstabilized on a mandrel having a corresponding outer diameter closelymatched thereto is preferred in order to minimize the amount ofshrinkage, to thereby provide for improved dimensional control.Therefore, although a uniform diameter (nontapered) biaxially orientedtubular member can be provided with a taper after the biaxial expansionmerely by shrinking onto a tapered mandrel during heat stabilization,such a method is much less preferred than a method in which the tubularmember is tapered during the biaxial expansion (e.g., by varying theaxial weight applied during the biaxial expansion within a taperedcapture member). Moreover, without a tapered capture member (e.g.,member 132), the range of possible dimensional combinations in thefinished tapered biaxially oriented tubular member is relativelylimited.

Thus, heat stabilization on a mandrel with a profile which closelymatches the as-expanded tapered biaxially oriented tubular member'sinner diameter not only provides the expected dimensional stabilitywhich comes from radial recovery of the expanded tube onto the mandrel,but also improved dimension control by avoiding inconsistencies in theas-expanded inner diameter. Specifically, the finished heat stabilizedtube has tight inner diameter tolerances that match those of the heatstabilization mandrel, and outer diameters that are less variable thanbefore the heat stabilization. For example, in one embodiment, taperedbiaxially oriented tubular member 123 has an inner diameter of about0.77+/−0.04 mm before heat stabilization, and of about 0.725+/−0.005after heat stabilization on a mandrel.

The heat stabilization mandrel has an outer diameter that is less thanthe inner diameter of the tapered biaxially oriented tubular member toallow the mandrel to be slid therein, but is preferably not more thanabout 10% less than the inner diameter of the as-expanded (before heatstabilization) tapered biaxially oriented tubular member.

To prepare a tapered biaxially oriented tubular member according to amethod of the invention, a combination of experimental and analyticalmethods can be used in order to predict the varying diameter/wallthickness of the resulting tapered biaxially oriented tubular member.The experimental and analytical methods take into account a number offactors such as the external axial load, capture member inner diameter,and the actual deformation behavior of the extruded tube 30 during itsbrief exposure to the moving heat source (nozzle 35). For example, anumber of samples of extruded tubes 30 were biaxially oriented withradially expansive gas pressure and a varying external weight, and thetotal axial stress was calculated and the actual axial expansion wasexperimentally observed. The total axial stress was calculated by firstcalculating the total axial load, which is the sum of the externalweight and the axial load due to the gas pressure within the tube 30.The axial load due to the gas pressure within the tube is based on theas-expanded inner diameter, which is calculated using the as-extrudeddimensions, the as-expanded outer diameter, and the axial expansionpercentage for a constant gas pressure of, e.g., 475 psig.

A second order polynomial was fit to a plot of the calculated totalaxial stress vs. observed axial expansion percentage, as illustrated inFIG. 13. The second order polynomial had an R-squared value of 0.993,and therefore may be used as an effective method for predicting axialexpansion percentage (and thus wall thickness) after biaxial expansion.The second order polynomial mathematically defines the relationshipbetween the stress and strain in the axial direction for a particularpolymer tube under a specific set of experimental conditions, and can beused to predict the external axial load necessary to produce a givenwall thickness during biaxial expansion into a known capture membersize.

To make the external axial load predictions, typically the as-extrudedouter and inner diameters are selected and the target final outer andinner diameter values are identified at various locations along thelength of the tubular member. The first step is then to determine therequired stretch percentages at locations of interest per the followingformula where OD₁ and ID₁ are as-extruded tube dimensions, and OD₂ andID₂ are the expanded dimensions at a given location:Stretch Percentage=100×[(OD ₁ ² −ID ₁ ²)/(OD ₂ ² −ID ₂ ²)−1]  Eq. 1)Using the calculated stretch percentage values, the associated totalaxial stress values (S) are then determined using the experimentallyderived polynomial, where a, b, and c are the coefficients obtained fromfitting a second order polynomial to experimental data (in FIG. 13 theyare 687.44, 1836.5, and 113.01, respectively):S(total)=a×(Stretch %)² +b×(Stretch %)+c  Eq. 2)Using the total axial stress determined via equations 1 and 2, thenecessary external weight (W) at each location can be predicted usingthe following equation:W=[S(total)−(P×ID ₂ ²)/(OD ₂ ² −ID ₂ ²)]×[(3.14159/4)×(OD ₂ ² −ID ₂²)]  Eq. 3)

The resulting prediction of external weight for each location ofinterest is then used to prepare a tapered biaxially oriented tubularmember having specific, desired dimensions. In one embodiment, theproduction is automated by programming the predicted external weightvalues into a closed loop feedback-controlled tension system so that theapplied external weight is controllably varied as a function of axiallocation (i.e., the position of the traveling heat source).Alternatively, an operator can manually adjust the applied externalaxial weight during the biaxial expansion process.

The following examples illustrate the formation of biaxially orientedtubular members in accordance with embodiments of the invention.

Example 1

PEBAX 63D was used to extrude four sets of multiple tubing samples (N=5)having an extruded inner diameter (ID) of about 0.005 inches, and anextruded outer diameter (OD) ranging from about 0.0217 inches to about0.0264 inches. The extruded tubing was placed inside a stainless steelcapture tube having a Teflon liner with an ID of about 0.034 inches, andradially and axially expanded therein at an elevated temperature.Specifically, a vertical hot air necking apparatus was used topressurize the tubing with pressurized air at about 500 psi and tosimultaneously lengthen the tubing with an axial load pulling on one endof the tubing, while the tubing was heated within the capture tube usinga heating nozzle traversing along the outside of the capture tube at aset point of about 385° F. (196° C.) (the temperature within the innerchamber of the capture tube is typically less than the set point, anddepends upon factors such as the nozzle temperature set point, thenozzle speed, the nozzle air flow rate, and the capture tube materialsand dimensions). The resulting biaxially oriented expanded tubularmember samples had similar final dimensions of about 0.0285 inch ID and0.033 inch OD, and a relatively high rupture pressure of not less thanabout 600 psi, and relatively low Gurley bending stiffness of about 102mg or less. The average longitudinal stretch percentage, and the meanrupture pressure, Gurley bending stiffness, and tensile load of theresulting tubular member samples, following stabilization at 100° C./15minutes on a 0.028-0.0285 inch mandrel, are given below in Table 1.

TABLE 1 Bending Mean Tensile Ex- Avg. Stiffness Rupture Break trudedExtruded Stretch Gurley Pressure Load ID (in) OD (in) (%) Units (mg)(psi) (lbf) Extrusion 0.0057 0.0217 85 97.2 665 2.28 Lot No. 1 (N = 5)Extrusion 0.0054 0.0235 113 102.2 697 2.56 Lot No. 2 (N = 5) Extrusion0.0053 0.0249 140 92.9 664 3.49 Lot No. 3 (N = 5) Extrusion 0.00570.0264 166 88.8 606 3.91 Lot No. 4 (N = 5)

Extruding a soft material such as PEBAX 63D directly to the finaldimensions (0.0285 inch ID, 0.033 inch OD) would be expected to producea tubular member having an unacceptably low rupture and tensile strengthfor use as the shaft tubular member. By way of comparison, tubularmembers of PEBAX 72D extruded directly to the final dimensions of about0.028 inch ID and 0.032 inch OD, and similarly stabilized at 100° C./15min., had a Gurley Bending Stiffness of about 223.1 mg, and a meanrupture pressure of about 436 psi. It should be noted that PEBAX 72D hasa higher durometer than the PEBAX 63D, so that the higher bendingstiffness is to be expected. Increasing the wall thickness in a secondset of PEBAX 72D comparison tubular members, which is expected toincrease the rupture pressure and bending stiffness of the tubing(specifically, the tubing had extruded dimensions of about 0.031 ID and0.037 inch OD, and was similarly stabilized at 100° C./15 min.),increased the mean rupture pressure of the comparison tubular members toabout 499 psi, but also (disadvantageously) increased the Gurley BendingStiffness to 258.6 mg. Although this bending stiffness would be expectedto decrease with a lower durometer material (e.g., PEBAX 63D), acorresponding decrease in the rupture pressure, with large radial growthprior to rupture, would also be expected.

Example 2

Equation 3 was used to predict the external weight values required tomake a variable outer diameter, constant inner diameter tubular membersuch as the tapered biaxially oriented tubular member 123 of FIG. 7. Thepredicted external weight values (using Eq. 3) are tabulated in Table 2along with the desired as-extruded and final expanded dimensions, thecalculated stretch percentages (using Eq. 1), and the calculated totalaxial stress values (using Eq. 2). In the example set forth in Table 2,the as-expanded outer diameter and wall thickness is to be held constantover the first 20 cm, and then increased linearly over the remaininglength. The internal pressure during the biaxial expansion taperingprocedure was constant at 475 psi.

TABLE 2 Axial Axial Stress External Location ID1 OD1 ID2 OD2 StretchS(total) Weight (cm) (cm) (cm) (cm) (cm) (%) (psi) (grams) 0 to 20 0.0160.074 0.073 0.0838 229.6 7956 560.6 22 0.016 0.074 0.073 0.0848 197.86433 487.0 24 0.016 0.074 0.073 0.0858 171.2 5272 423.9 26 0.016 0.0740.073 0.0868 148.8 4366 368.9 28 0.016 0.074 0.073 0.0878 129.5 3646320.3 30 0.016 0.074 0.073 0.0889 112.9 3063 276.7 32 0.016 0.074 0.0730.0899 98.4 2584 237.1 34 0.016 0.074 0.073 0.0909 85.5 2187 201.0 360.016 0.074 0.073 0.0919 74.2 1853 167.5 38 0.016 0.074 0.073 0.092964.0 1569 136.4 40 0.016 0.074 0.073 0.0939 54.8 1327 107.3

The as-expanded outer diameter set forth in Table 2 is the result of acapture member, such as tapered capture member 132, having an innerdiameter contoured to correspond to the desired expanded outer diameterof the biaxially oriented tubular member. The as-expanded inner diameteris to be held constant over the entire length by controllably varyingthe external weight. The predicted external weight is diminished fromapproximately 560 grams to approximately 107 grams, and the predicteddiminishment of weight is nearly linear. As a result, the actual weightdecline could be made linear with minimal impact on the resultingdimensions of the tapered section of the tubular member.

Example 3

To make a variable outer diameter, variable inner diameter, constantwall thickness tubular member such as the tapered biaxially orientedtubular member 135 of FIG. 10, the same conditions as in Example 2 areused, but the external weight decreases by a smaller amount from the 20cm to the 40 cm axial location. Specifically, to maintain a constantwall thickness of 0.004 inches (0.01 cm) as the outer diameter of thetube increases as in Example 2 from 0.0838 to 0.0939 cm, the axialstretch percentage, total stress, and external weight decrease as setforth in Table 3.

TABLE 3 Axial Location Axial Stretch Stress (S) (total) External Weight(cm) (%) (psi) (grams) 0-20 229.6 7956 560.6 22 225.4 7747 547.1 24221.4 7547 533.7 26 217.4 7353 520.4 28 213.5 7166 507.4 30 209.7 6986494.4 32 206.0 6812 481.06 34 202.3 6644 468.9 36 198.8 6481 456.3 38195.4 6324 443.8 40 192.0 6172 431.4

The bending moment of inertia (I) can be calculated using theengineering formula for a hollow cylindrical beam, I=(3.14159/64)×(OD₂⁴−ID₂ ⁴). FIG. 14 is a graph of the resulting calculated moment ofinertia values plotted as a function of nozzle position for Examples 2and 3. The graph includes also a comparison example in which the sameinitial external weight and tapered capture member profile as inExamples 2 and 3 would be used, but the external weight would be heldconstant throughout the biaxial expansion. The initial value of I isabout 2.35×10⁻⁸ for all three samples, and for Example 2 it increases toabout 5.728×10⁻⁸ whereas it increases a smaller amount to about3.378×10⁻⁸ for Example 3, and to about 2.951×10⁻⁸ for the fixed weightcomparison example.

Thus, as illustrated in FIG. 14, the greatest overall increase inbending stiffness occurs with the tapered biaxially expanded tubularmember of Example 2, in which the wall thickness increases along withthe increase in outer diameter of the expanded tubular member. Example 3has substantially less gain in stiffness along the tapered region of theexpanded tubular member, because its wall thickness is constant as theouter diameter increases. The stiffness profile for the fixed weightcomparison example has even less gain because, without decreasing theexternal weight to account for the rising outer diameter, the wallthickness of the expanded tubular member must become thinner as itsouter diameter increases. Specifically, the moment of inertia increasesby about 144% for Example 2, and about 44% for Example 3, and about25.5% for the fixed weight comparison example. Thus, the taperedbiaxially oriented tubular member of Example 2 can provide the greatestchange in stiffness for a fixed material durometer hardness and giventapering transition length, resulting in an improved transition from ahighly flexible distal shaft section to a substantially more rigidproximal shaft section of a catheter of the invention.

While the present invention is described herein in terms of certainpreferred embodiments, those skilled in the art will recognize thatvarious modifications and improvements may be made to the inventionwithout departing from the scope thereof. Moreover, although individualfeatures of one embodiment of the invention may be discussed herein orshown in the drawings of the one embodiment and not in otherembodiments, it should be apparent that individual features of oneembodiment may be combined with one or more features of anotherembodiment or features from a plurality of embodiments.

1. A balloon catheter, comprising a) an elongated shaft having aproximal section and a tubular member attached thereto with an inflationlumen extending there through, wherein said tubular member is formed ofa biaxially oriented nonporous thermoplastic polymer having a Shoredurometer hardness of less than about 75D and has at least a sectionwith a diameter tapering distally over a length of at least about 30% ofthe length of the tubular member such that the tapering section has abending stiffness that decreases distally by about 20% to about 80%therealong; and b) a balloon sealingly secured to said tubular member,which has an interior in fluid communication with the inflation lumen,and a rupture pressure which is less than the rupture pressure of theshaft tubular member.
 2. The catheter of claim 1 wherein the taperlength is greater than about 10 cm.
 3. The catheter of claim 1 whereinthe taper length is about 30 to about 65% of the length of the tubularmember.
 4. The catheter of claim 1 wherein the tapered section of thebiaxially oriented tubular member has a distally tapering wall thicknessand outer diameter, and the bending stiffness increases proximally by atleast about 65% therealong.
 5. The catheter of claim 1 wherein thetapered section of the biaxially oriented tubular member has a distallytapering inner diameter and a distally tapering outer diameter, and asubstantially constant wall thickness.
 6. The catheter of claim 1wherein the biaxially oriented tubular member bending stiffnessdecreases substantially linearly along the tapering section.
 7. Thecatheter of claim 1 wherein the biaxially oriented tubular member of theshaft is a distal outer member which has a distal end sealingly securedto the balloon and a proximal end sealingly secured to a proximaltubular member, and the bending stiffness decreases by a factor of about2 to 4.5 along the length of the tapering section.
 8. The catheter ofclaim 7 wherein the catheter is an over-the-wire catheter with aguidewire lumen proximal port at a proximal end of the catheter, and thetapered section has a proximal end at the proximal end of the biaxiallyoriented tubular member of the shaft.
 9. A balloon catheter, comprisinga) an elongated shaft having a proximal outer tubular member that isattached to a distal outer tubular member having an inflation lumenextending therein, an inner tubular member with a guidewire lumenextending therein, wherein said inner tubular member is disposed withinat least a portion of said inflation lumen, wherein said proximal outertubular member has a constant bending stiffness and said distal outertubular member is formed of a biaxially oriented nonporous thermoplasticpolymer having a Shore durometer hardness of less than about 75D, andwhich has a distal section, and a tapering proximal section with alength of at least about 30% of the length of the distal outer tubularmember and with a wall thickness tapering distally such that a bendingstiffness of the distal outer tubular member increases proximally by atleast about 65% along the tapering proximal section; and b) a balloonwhich has a proximal skirt sealingly secured to the distal outer tubularmember and a distal skirt secured to the inner tubular member such thatan interior of the balloon is in fluid communication with the inflationlumen, and a rupture pressure which is less than the rupture pressure ofthe distal outer tubular member.
 10. The balloon catheter of claim 9wherein the distal section of the biaxially oriented distal outertubular member extends from the tapering proximal section to the distalend of the biaxially oriented distal outer tubular member, and thedistal section of the biaxially oriented distal outer tubular member hasa constant inner and outer diameter and a constant wall thickness alongsubstantially its entire length.
 11. The balloon catheter of claim 9wherein the bending stiffness of the distal outer tubular memberdecreases by a factor of about 2 to 4.5 along the length of the taperingproximal section.
 12. The balloon catheter of claim 9 wherein thetapering proximal section of the biaxially oriented distal outer tubularmember has a constant inner diameter, and a distally tapering outerdiameter.
 13. The balloon catheter of claim 12 wherein the bendingstiffness of the distal outer tubular member increases proximally byabout 200 to about 350% along the tapering proximal section.
 14. Theballoon catheter of claim 9 wherein the length of the tapering proximalsection of the distal outer tubular member is at least about 10 cm. 15.The balloon catheter of claim 9 wherein the tapering proximal section ofthe biaxially oriented distal outer tubular member has a constant outerdiameter, and a distally increasing inner diameter.
 16. The ballooncatheter of claim 9 wherein the biaxially oriented tubular memberbending stiffness decreases substantially linearly along the taperingproximal section.
 17. The catheter of claim 9 wherein the biaxiallyoriented distal outer member has an axial orientation that increasesdistally along the tapering proximal section, and has a circumferentialorientation that is substantially uniform along the entire length of thebiaxially oriented distal outer member at least within a radial innersurface portion thereof.
 18. The catheter of claim 9 wherein the tubularmember has a Gurley bending stiffness of not greater than about 50 toabout 150 mg, a rupture pressure of at least about 20 to about 50 atm,and a tensile break load of at least about 1 to about 5 lbf.
 19. Thecatheter of claim 9 wherein the balloon is a relatively high pressureballoon having a rated burst pressure of about 14 to about 25 atm, andthe rupture pressure of the biaxially oriented tubular member is about20 to about 50 atm.